Speaking from "the other side" (IOW, having gone through what you have), I feel that there is a maturity difference between a 16-year-old and an 18-year-old. I'm not calling you "immature" by any means but it's been my observation that a 16-year-old in the throes of romance can become somewhat possessive and more than a little insecure.
By the time one is 16, one hasn't developed a "self-concept"--defining one's self in terms of one's self. Not "I am his girlfriend" or "I am somebody's daughter" but "I am a strong person and can accept (but not necessarily like) practically anything that comes my way".
When one gets older, one doesn't necessarily feel as though s/he has to be around the "beloved" all the time. The two are comfortable and secure enough in the relationship that they can be apart for a period of time (not defined) and know that everything is all right.
I was a couple of years younger than my Significant Other when I went away to Uni (and we're talking over 30 years ago). I was about three hours away and there was no way I could see him as often as you see yours. I was lucky if I saw him once every couple of months. It killed me in the beginning but I got used to it. I felt secure enough and he never gave me any inclination to feel otherwise.
Did you talk about this before he went away? If not, and this is really bothering you then the next time you see him you really need to discuss this. No accusations, nothing to make him defensive, just calmly explain to him your feelings. If he cares enough about you he'll do what he can to assuage your fears.
He's in a new atmosphere where he'll meet women his age and make friends with them. If that thought sends you into a frenzy then there is something seriously wrong. You can't expect that he'll cut himself off from 50% of the population whilst he's there no more than he would expect you to cut yourself off from 50% of the population.
I suggest that you find something else to occupy your mind--if you're in school, buckle down to your studies. Hang out with your friends, start an exercise regimen, just anything to get rid of the insecurity and the unhappiness that goes with it.
I hope this helps. Good luck and keep posting.